In the production of asphalt, the primary asphalt stock or starting material for producing the finished asphalt is usually derived primarily from vacuum tower bottoms. Although crudes may be selected to produce asphalts of a desired quality, petroleum asphalt is primarily a product of integrated refineries. As such, crudes are generally selected on the basis of the requirements of other products and the asphalt produced therefrom may vary somewhat in characteristics from one crude system to another. To achieve required properties, deasphalting unit tars and solvent extraction unit extracts are often used together with vacuum tower bottoms and other refinery streams, as blending stocks to make finished asphalts.
When asphalts are produced as a byproduct of a lubricant refinery, asphalt blending stocks may be produced at any of several of the various process units employed to remove unwanted components from the feedstock. Such process units include atmospheric and vacuum distillation towers, deasphalting units, solvent extraction units, dewaxing units, etc. As is well known to those skilled in the art, lube refineries do not manufacture a single base stock but rather process several distillate fractions and a vacuum residuum fraction. For example, the vacuum residuum fraction, after deasphalting, solvent extraction and dewaxing, is commonly referred to as "bright stock." Each generally subtractive step of the lube base stock refining process produces a byproduct which may be used as an asphalt blending stock, depending upon its characteristics, processed further or sold to an industry which has developed a use for the byproduct.
The conventional processing of crude oil to recover fractions suitable for upgrading operations employs multi-stage distillation. Crude oil is first distilled in a atmospheric distillation tower with residual material from the bottom of the distillation tower being further separated in a vacuum distillation tower. In this combination operation, gas and gasoline generally are recovered as overhead products of the atmospheric distillation tower, heavy naphtha, kerosene and gas oils are taken off as distillate side streams and the residual material is recovered from the bottom of the tower a reduced crude. The residual bottoms fraction or reduced crude is usually charged to a vacuum distillation tower. The vacuum distillation step in lube refining provides one or more raw stocks within the boiling range of about 550.degree. F. to 1050.degree. F., as well as the vacuum residuum byproduct In lube refining, excess liquid runback, known as overflash material, may be combined with the vacuum residuum and either withdrawn from the tower or charged to a deasphalting unit for further processing or dealt with in other conventional ways known to those skilled in the art. The overflash material may alternatively be withdrawn, recovered and charged directly to a solvent extraction unit. The presence of metallic impurities, asphaltenes and the like may render this material unsuitable for this step or likewise, for a catalytic processing step. Typical vacuum distillation systems are disclosed in U.S. Pat. Nos. 2,713,023, 3,886,062, 4,239,618 and 4,261,814, incorporated herein by reference in their entirety. Vacuum tower designs particularly germane to the present invention are disclosed in U.S. Pat. Nos. 3,929,626 and 3,989,616, which are hereby incorporated by reference in their entirety.
Following vacuum distillation, each raw stock is extracted with a solvent, e.g. furfural, phenol or chlorex, which is selective for aromatic hydrocarbons, removing these undesirable components. The vacuum residuum usually requires an additional step, typically propane deasphalting, to remove asphaltic material prior to solvent extraction. The products produced for further processing into base stocks are known as raffinates. The raffinate from solvent refining is thereafter dewaxed by admixing with a solvent such as a blend of methyl ethyl ketone and toluene, for example and then processed into finished base stocks.
The solvent extraction step separates hydrocarbon mixtures into two phases; the previously described raffinate phase which contains substances of relatively high hydrogen to carbon ratio, often called paraffinic type materials, and an extract phase which contains substances of relatively low hydrogen to carbon ratio often called aromatic type materials. Solvent extraction is possible because different liquid compounds have different solution affinities for each other and some combinations are completely miscible while other combinations are almost immiscible. The ability to distinguish between high carbon to hydrogen aromatic type and low carbon to hydrogen or paraffinic type materials is termed selectivity. The more finely this distinguishing can be done the higher the selectivity of the solvent.
Furfural is typical of a suitable solvent extraction agent. Its miscibility characteristics and physical properties permit use with both highly aromatic and highly paraffinic oils of wide boiling range. Diesel fuels and light and heavy lubricating stocks are refined with furfural. Furfural exhibits good selectivity at elevated temperatures (175.degree.-250.degree. F.). In a typical furfural solvent extraction unit for lubricating oils, the raw feed is introduced below or about at the center of the extraction tower. Furfural is fed into the top or upper portion of the tower. Recycled extract may be introduced into the lower section of the tower as reflux. Likewise, internal reflux is effected in the tower by the temperature gradient which is brought about by introducing the solvent at an elevated temperature and by intermediate cooling systems. Furfural solvent is recovered from the raffinate and extract phase streams or layers in suitable distillation and stripping equipment. The stripped and recovered solvent is then recycled.
While the furfural solvent extraction unit raffinate goes on to further processing, the extract from the operation often finds utility in a broad range of applications, including use as an asphalt blending stock, as indicated above. Bright stock extracts (BSE's), obtained by solvent-refining deasphalted vacuum resids during the production of bright stocks, are also useful in rubber processing and find utility as ink oils as well.
In recent years, concerns have arisen regarding the potential hazards associated with the use of asphalts and various blending stocks employed in the formulation of asphalts. For example, U.S. Pat. No. 4,139,511 notes that a particular paving material used in the paving of roads and airfields which "contains a large amount of a carcinogenic substance benzo(a)pyrene". U.S. Pat. No. 4,139,511 has as an object thereof to provide a uniform and satisfactorily hard asphalt composition containing little or no carcinogenic substance".
Asphalt products have heretofore been designed on the basis of engineering criteria only, such criteria including viscosity, ductility, penetration, etc. Only recently, however, has attention been directed toward the potential toxicity of asphalt and asphalt-based products. The toxicity issue of recent concern relates to the potential carcinogenicity of fumes generated from asphalt during end-use applications such as during paving an roofing operations.
To determine the relative carcinogenic activity of a asphalt composition or an asphalt blending stock such as a vacuum tower residuum, a propane deasphalting unit tar or an aromatic extract, a reliable test method for assaying such activity in complex hydrocarbon mixtures is required. A highly reproducible method showing strong correlation with the carcinogenic activity index of hydrocarbon mixtures is disclosed in U.S. Pat. No. 4,499,187, which is incorporated by reference in its entirety. From the testing of hydrocarbon samples as disclosed in U.S. Pat. No. 4,499,187, a property of the sample, known as its Mutagenicity Index (MI) is determined. Hydrocarbon mixtures exhibiting MI's less than or equal to 1.0 are known to be non-carcinogenic, while samples exhibiting MI's equal to about 0.0 are known to be completely free of mutagenic activity. As can be readily appreciated, it would be desirable to produce blending stocks for asphalts which are non-carcinogenic such that contact with same will not cause the development of cancerous growths in living tissue. It would be still more desirable to produce such materials which are free of mutagenic activity; that is, that contact with such products would not induce mutations in DNA and in living cells.
Therefore, what is needed is a process for the production of non-carcinogenic asphalts and asphalt blending stocks for use in roofing, paving and all other applications for which asphalt and bitumen are used.